Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks comprising ethane or ethane/propane mixtures, known as gas cracking, or propane, butane, naphtha, NGL (natural gas liquids), condensates, kero, gas oil and hydrowax, known as naphtha cracking. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into, for instance, methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process.
In an OTO process, an oxygenate such as methanol or dimethylether is provided to a reaction zone of a reactor comprising a suitable conversion catalyst and is converted to ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the oxygenate such as methanol is converted to higher hydrocarbons including C4+ olefins, paraffins and carbonaceous deposits on the catalyst. The catalyst is continuously regenerated to remove a portion of the carbonaceous deposits by methods known in the art, for example heating the catalyst with an oxygen-containing gas such as air or oxygen.
The effluent from the reactor, comprising olefins, any unreacted oxygenates such as methanol and dimethylether and other reaction products such as water, once separated from the bulk of the catalyst, is then treated to provide separate component streams. In order to increase the ethylene and propylene yield of the process, the C4+ olefins component stream may be recycled to the reaction zone or alternatively further cracked in a dedicated olefin cracking zone to produce further ethylene and propylene.
Following reaction, the reaction effluent stream is cooled and must be separated into its components, including the desired olefinic products. After initial indirect cooling, for example in a heat exchanger, the reaction effluent stream is contacted with a cooled aqueous stream in a quench zone. Water and most of the oxygenates present will be separated in the quench zone.
However, some of the oxygenates present may be carried over in the olefin rich gas stream from the top of the quench zone. These oxygenates (for example methanol, DME, aldehydes, such as formaldehyde, acetaldehyde and propionaldehyde and ketones, such as methylethylketone) will then need to be removed at a later stage in the separation process to prevent them being present as contaminants in the final product.
In known processes for the production of olefins from oxygenates, the olefin rich gas stream from the quench zone is compressed. The compression is carried out in stages by one or more compressors in series. After each compressor, the compressed gas stream must be cooled and this is usually carried out by indirect heat exchange using an air or water heat exchanger. A separation vessel, such as a knock out drum, is situated after each heat exchanger to separate any condensed materials from the compressed and cooled gas stream. During or after the compression and cooling process, carbon monoxide and carbon dioxide formed as by-products in the OTO reaction zone, are removed from the gas stream in a carbonyl compound removal zone, for example by treating the gas stream with a caustic solution.
The presence of certain oxygenates at this stage can cause problems when treating the gas stream with a caustic solution, as the basic components of the caustic solution, such as hydroxide ions, can catalyse the aldol condensation and subsequent dehydration reactions of particularly acetaldehyde to form unsaturated aldehydes such as acrolein, especially at higher pH, such as a pH of greater than 9. Unsaturated aldehydes will polymerise when allowed to accumulate in the caustic solution and, if the aldol condensation reaction is left unchecked, viscous oily polymers and polymer films and lumbs can be formed. These are known as ‘red oil’, are insoluble in the caustic solution and can deposit on equipment internals, causing severe fouling and blockages.
In prior art processes, oxygenates remaining in the olefin rich gas stream resulting from the quench zone are usually removed using an alcohol, usually methanol, wash or by extractive distillation with an alcohol. These processes take place after at least one complete compression stage comprising compression, cooling in a heat exchanger and separation of condensed material. Examples of alcohol wash processes in the prior art can be found in US 2005/0033104, US 2005/0222478, US 2006/0004239 and US 2005/0283038. An extractive distillation process is described in US 2003/0045655.
It would be desirable to provide a simple, integrated process for the removal of oxygenates from the olefin rich gas stream produced in the quench zone and the cooling of said gas stream.